1. Field of the Invention
The subject invention relates to an exhaust muffler having a stamp formed outer shell defined by laminated sheets which have a minute air gap therebetween and which are hermetically sealed around their peripheries.
2. Description of the Prior Art
Most prior art exhaust mufflers include an array of pipes supported in parallel relationship by a plurality of transversely extending baffles. The baffles typically are of identical oval or circular cross-sectional shape. A sheet of metal is then wrapped around the baffles to define an open ended shell with a circular or oval cross-sectional shape conforming to the shape of the baffles. Opposed longitudinal edges of the shell are mechanically folded in overlapped relationship to secure the shell around the baffles.
An outer shell formed from a single thickness of metal may vibrate in response to the pulsating flow of exhaust gas through the muffler. These shell vibrations are known to create a shell ring independent of the noise associated with the flowing exhaust gas. Hence, a second sheet of metal may be wrapped around the shell, and opposed longitudinal edges may be engaged with one another to define an outer wrapper. The outer wrapper dampens the vibrations of the shell and attenuates shell ring.
Opposed end caps or headers which conform to the shape of the baffles are mechanically locked to opposed ends of the shell and wrapper. Each header includes at least one aperture which enables exhaust gas to flow into and out of the muffler.
Many prior art mufflers include an array of embossments on either the shell or the wrapper. The shell and the wrapper will contact one another only at the embossed regions. Remaining regions will define spaces that are intended to enhance noise insulation.
The prior art also includes exhaust mufflers that are defined, at least in part, by stamp formed sheets of metal. For example, some prior art mufflers include conventional tubes and baffles surrounded by opposed stamp formed outer shells. The shells are formed to define peripheral flanges that can be registered with one another to substantially enclose the tubes, and baffles. The registered flanges on the prior art muffler have been attached by mechanical crimping or by spot welding. The spot welds typically are placed close to one another to effectively define a seam around the periphery of the muffler. An example of a muffler of this type is shown in published U.K. Patent Application No. 2,120,318 dated Nov. 30, 1983.
Other prior art mufflers include only a pair of plates that are stamped to define an array of channels and chambers. The plates are secured in face-to-face relationship by welding or crimping, and the stamp formed channels and chambers define a flow path for exhaust gas that will attenuate noise. A muffler of this general type is shown in U.S. Pat. No. 3,638,756 which issued to Thiele on Feb. 1, 1972.
The prior art also includes mufflers with stamp formed plates to define tubes and stamp formed outer shells to define chambers. A commercially successful muffler of this type is shown in U.S. Pat. No. Re 33,370 which was reissued on Oct. 9, 1990.
Stamp formed outer shells on exhaust mufflers also can generate vibration related noise. Shell vibrations in stamp formed mufflers typically have been attenuated by stamping an array of reinforcing ribs in the external shell. One particularly effective pattern of reinforcing ribs is shown in U.S. Pat. No. 4,924,968 which issued on May 15, 1990. Although ribs can be effective in attenuating the vibration of shells, the ribs also add significantly to the total amount of metal required for the outer shell. Additionally, ribs can complicate the stamping process and can generate excessive local stretching of metal. Ribs also can be difficult to design. A rib pattern that is effective for one muffler may not necessarily be effective for a different muffler, and engineers must experiment with different rib dimensions and spacings for each family of similar mufflers.
Prior art mufflers also have been provided with stamp formed heat shields, as shown, for example, in U.S. Pat. No. 4,759,423 is which issued on Jul. 26, 1988. One heat shield shown in U.S. Pat. No. 4,759,423 is formed on the same stamping apparatus as the corresponding external shell and then is nested over the muffler. Another heat shield shown in U.S. Pat. No. 4,759,423 is formed on a different stamping apparatus and is formed to have selected areas spaced significantly from the associated external shell. Both embodiments show the heat shield being formed from a different material than the external shells. Spot welding through more than four thicknesses of metal is very difficult. Hence, heat shields of the type shown in U.S. Pat. No. 4,759,423 would be attached after the muffler was completed.
Exhaust mufflers are susceptible to corrosive damage. Corrosion can be generated from either the inside of the muffler or the outside. For example, exhaust gases contain both corrosive chemicals and moisture. The mixture of vapors and corrosive chemicals often condenses at a low point of a muffler. This corrosive sludge may act on and eat through the metallic shell of the muffler. The prior art includes several approaches for reducing internally generated corrosion. For example, it is well known to place a small drain hole at the low point of a muffler. The prior art also includes siphon tubes having one end in an exhaust pipe and the opposed end at a location that is likely to accumulate moisture. The flowing exhaust gas effectively sucks moisture from the chamber and prevents a long term build-up. The prior art also attempts to avoid corrosion by using costly corrosion resistant metals when necessary.
Externally generated corrosion is common in northern climates where salt or other de-icing chemicals are used on roads. These chemicals can accumulate on horizontal surface and can accelerate corrosion. Once again, outer shells of mufflers can be formed from or coated with material that will resist corrosion from such de-icing chemicals.
Corrosion has been a particular problem in prior art mufflers having two layers of metallic material in close proximity to one another, such as the above described conventional mufflers with wrapped outer shells. Internally or externally generated corrosive chemicals are known to wick into microscopically small gaps between the mechanically folded seams on wrapped outer shells or between spot welded shields and external shells of mufflers. Corrosive liquids in these locations do not readily drain. Additionally, the corrosive liquid may function as an electrolyte with contributes to galvanic action between closely spaced layers of metal. Muffler manufacturers prefer the ability to use disparate metals for different parts of a muffler depending upon system needs. However, the galvanic reactions are greater when a corrosive chemical is disposed between two different metals. As a result, engineers often are forced to use two identical metals for a shell and a wrapper, even though two different metals might be more appropriate in other respects. Thus, for example, it may be required to use a more costly stainless steel for an outer wrapper even though a less costly low carbon steel would be functionally acceptable.
Seam welding has been used on many prior art stamp formed mufflers as an alternative to mechanical crimping of the peripheral flanges. Seam welding, however, is actually a series of closely spaced spot welds. Invariably, a plurality of microscopic gaps exist in a seam weld at spaced apart locations around the peripheral flanges. These small gaps do not affect acoustical performance. However, the gaps may permit wicking of corrosive liquids. Seam welding also is not well suited to connecting disparate metals. Hence, mufflers with welded seams typically use the same metal throughout.
In view of the above, it is an object of the subject invention to provide an exhaust muffler with an enhanced ability to eliminate shell ring.
It is another object of the subject invention to provide an exhaust muffler with enhanced heat insulating capabilities.
A further object of the subject invention is to provide an exhaust muffler which eliminates or reduces the need for reinforcing ribs in an outer shell.
An additional object of the subject invention is to provide an exhaust muffler with enhanced resistance to corrosion.
Yet a further object of the subject invention is to provide an improved method for manufacturing an exhaust muffler.